In recent years, it has been increasingly demanded from the viewpoint, for instance, of petroleum depletion and other global environmental issues, that an alternative fuel be used with internal combustion engines. Therefore, there is a pressing need to develop an internal combustion engine that operates on a wide variety of fuels. To permit an internal combustion engine to operate on a wide variety of fuels, it is preferred that the cetane number of a fuel be accurately detected in a vehicle. The reason is that combustion can be properly controlled in accordance with the type of an employed fuel when engine control is exercised in accordance with the fuel's cetane number.
A prior art device disclosed, for instance, in JP-A-2007-64157 detects actual ignition timing from a cylinder pressure detected by a cylinder pressure sensor, calculates an ignition delay correction amount so that the actual ignition timing agrees with target ignition timing, and estimates a cetane number from the ignition delay correction amount.    Patent Document 1: JP-A-2007-64157    Patent Document 2: JP-A-2007-46592    Patent Document 3: JP-A-2006-16994    Patent Document 4: JP-A-2005-344557